The ethylene cracking techniques used in petrochemical ethylene equipments mainly include those developed by LUMMUS Co. (USA), Stone & Webster Co. (USA), Kellog & Braun Root Co. (USA), Linde Co. (Germany), Technip KTI Co. (Netherlands), and the CBL cracking furnace developed by China Petrochemical Corporation.
FIG. 1A shows a typical ethylene cracking furnace 10, which comprises a radiant section 11, a convective section 13, and a flue section 12 located between the radiant section 11 and the convective section 13. Within the radiant section 11, a radiant coil 14 is provided in the central plane P of the radiant section 11 along the longitudinal direction thereof. In addition, the radiant section 11 is further provided with bottom burners 15 and/or side burners 16 for heating. Moreover, the ethylene cracking furnace 10 further comprises a transfer line exchanger 17, a high-pressure steam drum 18, and an induced draft fan 19, etc.
To significantly reduce the feedstock consumption, maintain a suitable run length, and have a good feedstock flexibility, nowadays a two-pass high-selectivity radiant coil with or without branches of variable diameters is used. The first-pass tube of the radiant coil is of a small diameter. Therefore, a quick temperature rise can be achieved since the specific surface area of a small-diameter tube is relatively large. The second-pass tube is of a large diameter, in order to reduce the influences on coking sensitivity. The two-pass radiant coil can be configured as 1-1 type (U type), 2-1 type, 4-1 type, 6-1 type coil, etc.
A two-pass 1-1 type coil structure, which can be matched to transfer line exchanger(s), is of a large specific surface area and good mechanical performance. The run length thereof, however, is slightly short.
For an N−1 (N>1) type coil structure, the number of tubes in the first pass is N times as more as the number of the tubes in the second pass. Therefore, the N tubes in the first pass need to be combined into one tube before being connected to a corresponding second-pass tube. EP 1146105 discloses a cracking furnace having a two-pass 2-1 type coil structure. As indicated in FIG. 1B, a two-pass radiant coil comprises first-pass tubes 51 (16 tubes) and second-pass tubes 52 (8 tubes) perpendicularly arranged in an inner chamber of a radiant section. All these tubes are located in one common plane, with all the first-pass tubes 51 arranged together, and all the second-pass tubes 52 arranged together, wherein every two first-pass tubes 51 are combined into one tube by a Y-shaped manifold 53 at a lower portion of the first-pass tubes 51 before being connected to a second-pass tube 52 via two S-shaped elbows 54 and a U-shaped elbow 55.
CN 1067669 discloses a cracking furnace having a two-pass 6-1 type coil structure, which includes 6 first-pass tubes, and one second-pass tube. Similarly, these 6 first-pass tubes are first combined into one tube via a rigid manifold arranged in a lower portion thereof, and then are connected to the second-pass tube.
In the above structures, since the number of the first-pass tubes is a plurality of times higher than the number of the second-pass tubes, when the coil is heated to expand, the second-pass tubes first expand downward, and then the first-pass tubes are dragged by the second-pass tubes to move downward also, wherein the first-pass tubes are easily bent because they are deformed under different forces. The rigidity of the manifold connected in the lower portion of the first-pass tubes prevents expansion differences thereof from being absorbed by an S-shaped tube (if any), rendering the coil easily being bent. Hence, the mechanical performance of the coil is reduced, thereby shortening the service life of the coil and the run length of the cracking furnace.